Adds wide gamut framework breakage notice.

[gaaclarke]

Description of what this PR is changing or adding, and why:

Adding breaking change notice for wide gamut colors in the framework.

Issues fixed by this PR (if any):

https://github.com/flutter/flutter/issues/127855

PRs or commits this PR depends on (if any):

https://github.com/flutter/engine/pull/54737

Presubmit checklist

• [x] This PR is marked as draft with an explanation if not meant to land until a future stable release.
• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.